1. Field of the Invention
This invention relates to the use of non-hydrated iron oxides in permeable beds for reacting hydrogen sulfide from relatively dry gas, with minimal added moisture to maintain reactivity, and to restoring the beds to full reactivity after the bed moisture has been used up.
2. Description of Related Art
In removing hydrogen sulfide from natural gas by reacting with iron oxides, it is known from field experience that H.sub.2 O must be present. The gas produced by most wells contains adequate water vapor. However, premature loss of oxide reactivity may occur in at least the following situations:
1. Where, atypically, the gas is so lacking in moisture as to be considered "under-saturated"; and especially
2. Where relatively dry gas is greatly chilled before being reacted, to remove those heavier hydrocarbons likely to form droplets in the outlet gas; as these are removed by chilling, it precipitates the water vapor as well.
In reacting hydrogen sulfide in gas by flowing it through a permeable bed, the iron oxide heretofore conventionally used in the bed is the hydrated oxide Fe.sub.2 O.sub.3 .times.H.sub.2 O. In the well-known wood chip process, a pervious bed of moistened wood chips is formed, through which this hydrated oxide is intermixed. In use, when substantially dry gas flows through the bed of oxide and moistened wood chips after the bed's moisture has dried, the water of hydration incorporated in the oxide will dry also, suddenly rendering the oxide incapable of reacting further. Adding water to the bed at this stage serves no function; the reactivity of the oxide has been destroyed. The sweetening process is then at a halt, the old bed must be removed from the reactor vessel, and a new bed of reactive material provided. If at this time a substantial portion of the reactive capacity of the Fe.sub.2 O.sub.3 has been utilized, it will have solidified or "cemented" the bed, making its removal difficult and time-consuming.
To avoid premature shut-down of the sweetening process, water has been precautionarily added to the inflowing gas throughout the process, in amounts which are more than necessary to continue the reactivity of the oxide. Such excess water must be removed downstream, adding to the expense and complexity of the process.